1. Field of the Invention
The present invention relates to an image intensifier for converting an incident light image into a visible light image.
2. Description of the Related Art
In an X-ray diagnosis system for medical use, a nondestructive inspection device for industrial use, and an ultraviolet rays detection system for space observation, each of which uses an image intensifier, an image by X-rays, ultraviolet rays, neutron rays or the like, which has transmitted through an object, is converted into a visible light image by the image intensifier. The visible light image is then picked up by an image pickup camera, and this picked up image is visually presented to viewers on a monitor.
A conventional image intensifier includes a vacuum envelope having an input window located on the side which receives X-rays or the like and an output window on the opposite side to the input window. Within the vacuum envelope, an input surface which converts X-rays or the like into an electron beam and emits the electron beam, is provided on the inner side of the input window, and an output surface which converts the electron beam into a visible light image and outputs the image is provided on the inner side of the output window. Further, an electron lens for accelerating or condensing the electron beam is provided along the path of the electron beam which travels from the input surface to the output surface. The electron lens includes a cathode for applying negative voltage to the input surface, an anode for applying high positive voltage to the output surface, and a plurality of grid electrodes located between the cathode and the anode.
When high voltage for bulb driving purpose is applied to the image intensifier thus constructed, a potential difference, for example, between the grid electrode and the anode reaches 6 kV/mm. The grid electrode is easy to emit electrons at such a portion that the intensity of the electric field is high and the potential gradient is high. When metal foreign matter is present on the grid electrode, the likelihood of electric field emission further increases. The heat caused by the electron emission causes the grid electrode to generate gas. The gas is ionized by the electrons and the generated ions collide with the grid electrode to emit secondary electrons. As a result, the local abnormal discharge continues and reaches the input surface. The discharge causes the photoelectric layer to emit unwanted photoelectrons, the photoelectrons hit the output surface, and in turn the output surface fluoresces. This forms a major cause for a so-called unwanted fluorescence of the image intensifier. The unwanted photoelectrons cause the potentials at those electrodes to vary to make the operation of the image intensifier unstable.
An effective measure for those problems is to cover the portion having the potential gradient, including the grid electrodes, with a material which has a low secondary electron emission coefficient but a certain level of conductivity. A typical example of the material is a chromium oxide film (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 58-5319, pages 1 to 2, FIG. 1).
In the case of the existent chromium oxide film, the adhesive force of the film to the electrode or the like is poor and the interparticle binding force is also poor. Further, the chromium oxide film is easy to be separated by vibration and impact in the manufacturing stage or when it is used or when ambient conditions abruptly change. When the chromium oxide film is separated, the secondary electrons are emitted from the portion from which the chromium oxide film has been separated. This brings about the unwanted fluorescence and the unstable operation. Additionally, the separated film pieces are present as foreign matter in the bulb. This leads to defective products, lowering production yield and product quality. A known technique to increase the adhesive force and the interparticle binding force is to add liquid glass or the like as a binder to the chromium oxide film. This technique has the following disadvantages, however. The conductivity of the chromium oxide film is easily lost. The secondary electrons are less emitted, but the high electric insulation brings about the charging of the film, causing dust attraction and unstable potential distribution in the bulb.
To cope with this problem, there is a proposal in which a composition ratio of the chromium oxide film is set at 25 to 40 atom % of chromium, 1 to 8 atom % of silicon, 0.7 to 5 atom % of alkali metal, and the remaining part of the content substantially consisting of oxygen. When the chromium oxide film has such a composition ratio, the following advantages are produced. Proper conductivity of the film and low secondary electron emission are secured with no dust attraction and no unwanted fluorescence. Its adhesive force to the film forming portion and the interparticle binding force are satisfactorily secured to prevent the film separation. As a result, secondary electron emission due to the film separation and product defectiveness due to the foreign matter in the bulb are successfully prevented.
In the case where metal foreign matter is present, it may be a discharge source even in a location where the potential difference between the grid electrode and the anode is far below 6 kV/mm.
The metal foreign matter is produced by burr produced at the time of working the electrodes, the rubbing of the electrodes when assembled into the bulb, at the time of welding, and the like. The metal foreign matter may be put out of the bulb in certain levels by removing the burr, improving the assembling process, modifying the welding conditions to reduce the likelihood of performing the welding work in the bulb, and further by tapping and cleaning the inside of the bulb. Even when those approaches to remove the metal foreign matter are used, it is almost impossible to completely remove the metal foreign matter from the bulb.
The metal foreign matter is made of SUS, AL, Cu and the like and sometimes takes the form of needle 50 to 200 microns long. A coulomb force acts on the metal foreign matter of such a size in the electric field of 0.5 kV/mm or higher, and the metal foreign matter moves around. The following fact was experimentally confirmed: when the image intensifier is operating, metal foreign matter having been present in the bulb is placed on the grid electrode, and behaves to rise and float toward the anode by Coulomb force. An electric field concentrates at the metal foreign matter, discharging current flows, and the metal foreign matter is molten to bond to the grid electrode. The discharging is continuously performed and eventually the image intensifier is damaged to be inoperable.
This problem was successfully solved in such a manner that the chromium oxide film is formed at such a location of the electrode where the electric field is 0.5 kV/mm or higher. 0.5 kV/mm of the electric field is a critical value at which the metal foreign matter is allowed to move around under the Coulomb force. If the grid electrode is protected by the chromium oxide film, even when the metal foreign matter rises by the Coulomb force and the electric field concentrates thereat, the discharging is prevented. Even if the discharging occurs, the metal foreign matter is not molten to bond to the grid electrode and thus no serious continuous discharge occurs (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2005-268197, page 4, FIG. 1).
As mentioned above, the problem of the continuous discharging phenomenon caused by the metal foreign matter was substantially solved. Through calculations, experiments and trial production, it has been found that an unwanted intermittent discharging phenomenon occurs from the spaces between a plurality of electrodes and the insulating member for insulating those electrodes.
It was found that the intermittent discharging phenomenon was due to an intermittent arcing occurring in the interface between the spaces between the electrodes and the insulating member for insulating those electrodes. Even in the case of the typical 9-inch image intensifier, high voltage of 27 kV is applied to between the anode and the grid electrode functioning as an expanding electrode. Usually, those are both insulatingly supported by the glass bulb of the vacuum envelope, for example. Electrons emitted from the grid electrode under the electric field negatively charge the glass bulb and the potential difference between the glass bulb and the anode gradually increases. When the potential difference exceeds a threshold value for the dielectric breakdown, arcing occurs in the interface between the glass bulb and the anode. The arcing light enters the input surface to cause the photoelectric surface to emit unwanted photoelectrons, and then the unwanted photoelectrons cause the output surface to wrongly fluoresce. Then, the charging mentioned above starts, arcing occurs, and the output surface fluoresces. Repeating of such a process leads to the intermittent discharge phenomenon. The interval of the intermittent discharge phenomenon varies depending on the bulb structure, applied voltage and the like. Generally, it ranges from several hundred milliseconds to several hundred seconds. The phenomenon lowers the diagnosis level in the medical field and the nondestructive inspection field.